1. Field of the Invention:
The present invention relates to a coordinate input device using a matrix-type display panel.
2. Description of the Related Art:
In recent years, as an input device for an information apparatus such as a computer and an electronic organizer, a tablet, a touch-sensitive panel, and the like are replacing the conventional keyboard and scanner. For example, U.S. Pat. No. 4,848,496 describes an electromagnetic induction type tablet used as an input device for a computer using a detective pen as an input means, where a display device is disposed on the input device. U.S. Pat. No. 4,831,568 describes a tablet having an analog resistance film used as an input device for an electronic organizer, where a display device is disposed under the tablet. The number of such information apparatuses has increased.
However, the above information apparatuses have disadvantages. The cost increases since they include an input device and a display device separately. Further, the input device and the display device must be precisely aligned with each other, which results in the increase in the number of manufacturing steps. Also, the size and weight of the input device and the display device may be critical for a portable information apparatus. When a tablet having a resistance film is used as the input device, the input device is disposed on the display device as described above. In this case, since the transmittance of the tablet is as low as 7.0%, the entire transmittance reduces. As a result, the luminance of the display device with the tablet disposed thereon is only approximately 80% of that of the display device without the tablet thereon. It is possible to increase the luminance by increasing the luminance of a back light used. However, since a portable information apparatus is driven by a battery which necessitates a low power consumption, there is a limitation in increasing the luminance of the back light. For these reasons, only a binary display (black and white) is allowed for an information apparatus using a tablet with a low transmittance, making it difficult to realize a gradation display.
In order to overcome the above problem, Japanese Laid-Open Patent Publication Nos. 54-24588, 62-180417, and 2-255911, and U.S. Pat. No. 4,839,634 disclose matrix-type display panels used both for a display device and a coordinate input device. For example, in a conventional tablet integrated with a display device (hereinafter, such a tablet is referred to as a display-integrated tablet) described in Japanese Laid-Open Patent Publication No. 2-255911, a scanning pulse is applied to driving electrodes disposed in a display panel, and the scanning pulse is detected through capacitances between the respective driving electrodes and a detective pen. The detective pen is electrically connected to a detection circuit of the display-integrated tablet. When the maximum output is detected at one of the driving electrodes, the position corresponding to the driving electrode is determined to be the coordinate of the position touched by the detective pen.
Thus, the coordinate can be detected without disposing a separate input device on the display device. In such a conventional display-integrated tablet, however, it is difficult to effect the input using a finger instead of the detective pen. Using the finger is easier and more convenient than using the detective pen for an application including only menu selections. The coordinate input with the finger is conducted as follows: The operator touches a position on the display with a finger while simultaneously contacting a portion of his or her body other than the finger with an input terminal of a signal detection circuit incorporated in the display-integrated tablet. A signal is then transmitted from the finger to the input terminal through the body. The coordinate is detected based on the received signal. This input with the finger is however disadvantageous in that the resistance inside the human body is high and that noise such as an ambient electromagnetic wave and the like enters the signal during the transmission of the signal through the human body. As a result, the precision of the coordinate detection becomes low.
For the conventional display-integrated tablet, various types of noises enter the signal for the coordinate detection even when the detective pen is used. These noises are caused because the same panel is used both for the display and the coordinate input. Noises from a back light and an inverter disposed in the device is especially influential, reducing the resolution of the coordinate detection. Further, the frequency of the inverter is very close to that of the signal to be detected. It is therefore difficult to eliminate a noise generated from the inverter by using a filter or the like. A tight shield is required to eliminate such a noise. This causes cost increase and difficulty in designing the entire apparatus.